Liquid crystal display device incorporating phase difference plate and liquid crystal layer capable of improving viewing angle dependence

ABSTRACT

A liquid crystal display device incorporates two phase difference plates stacked together and disposed between a liquid crystal display element and a pair of polarizers, the liquid crystal display element being formed by sandwiching a liquid crystal layer between a pair of electrode substrates, and the polarizers being disposed on one side of the liquid crystal display element. Since the principal refractive indices n a , n b , and n c  of the phase difference plates are such that n a &lt;n b &lt;n c , the phase difference plates exhibit positive refractive index anisotropies. The principal refractive index n b  inclines to the normal to the surfaces of the phase difference plates about one of the principal refractive indices n a  or n c  that is parallel to the surfaces. In addition, the refractive index anisotropy an of the liquid crystal material constituting the liquid crystal layer is specified to vary with wavelengths of rays of light within a range that allows no viewing-angle dependent coloration to occur on a liquid crystal screen. This eliminates viewing-angle dependent phase differences that, otherwise, would occur to the liquid crystal display element, and especially prevents the coloring phenomenon efficiently on the liquid crystal screen that, otherwise, would, occur with larger viewing angles.

FIELD OF THE INVENTION

The present invention relates to a liquid crystal display device havinga display screen with viewing-angle characteristics improved by a phasedifference plate combined with a liquid crystal display element.

BACKGROUND OF THE INVENTION

Liquid crystal display devices using nematic liquid crystal displayelements, which have conventionally been widely used asnumeric-value-segment-type display devices such as watches and portablecalculators, have recently been also used in word processors, note-typepersonal computers, car-use liquid crystal televisions, and otherapparatuses.

Generally, a liquid crystal display element has a light-transmittingsubstrate and electrode lines for turning on and off pixels and othercomponents that are formed on the substrate. For example, in anactive-matrix liquid crystal display device, active elements, such asthin-film transistors, are formed on the substrate together with theelectrode lines as switching means for selectively driving pixelelectrodes by which voltages are applied across the liquid crystal.Further, in liquid crystal display devices capable of color display,color filter layers having colors such as red, green and blue areprovided on the substrate.

Liquid crystal display elements such as that mentioned above adopt aliquid crystal display mode that is suitably selected depending on twistangles of the liquid crystal: some of well-known modes areactive-driving-type twisted nematic liquid crystal display mode(hereinafter, referred to as the TN mode) and the multiplex-driving-typesuper-twisted nematic liquid crystal display mode (hereinafter, referredto as the STN mode).

The TN mode displays images by orienting the nematic liquid crystalmolecules to a 90°-twisted state so as to direct rays along the twisteddirections. The STN mode utilizes the fact that the transmittance isallowed to change abruptly in the vicinity of the threshold value of theapplied voltage across the liquid crystal by expanding the twist angleof the nematic liquid crystal molecules to not less than 90°.

The problem with the STN mode is that the background of the displayscreen sustains a peculiar color due to interference between colorsbecause of the use of the birefringence effect of liquid crystal. Inorder to solve this problem and to provide a proper black-and-whitedisplay in the STN mode, the application of an optical-retardationcompensation plate is considered to be effective. Display modes usingthe optical-retardation compensation plate are mainly classified intotwo modes, that is, the double layered super-twisted nematicoptical-retardation compensation mode (hereinafter, referred to as theDSTN mode) and the film-type optical-retardation compensation mode(hereinafter, referred to as the film-addition mode) wherein a filmhaving optical anisotropy is provided.

The DSTN mode uses a two-layered construction that has display-useliquid crystal cells and liquid crystal cells which are oriented with atwist angle in a direction reversed to that of the display-use liquidcrystal cells. The film-addition mode uses a construction wherein filmshaving optical anisotropy are placed. Here, the film-addition mode hasbeen considered to be more prospective on the standpoint of light weightand low costs. Since the application of such an optical-retardationcompensation mode makes it possible to improve the black-and-whitedisplay characteristics, color STN liquid crystal display devices, whichenable color display by installing color-filter layers in STN-modedisplay devices, have been achieved.

The TN modes are, on the other hand, classified into the normally blackmode and the normally white mode. In the normally black mode, a pair ofpolarization plates are placed with their polarizing directions inparallel with each other, and a black display is provided in a statewhere no on-voltage is applied across the liquid crystal layer(off-state). In the normally white mode, a pair of polarization platesare placed with their polarizing directions orthogonal to each other,and a white display is provided in the off-state. Here, the normallywhite mode is considered to be more prospective from the standpoints ofdisplay contrast, color reproducibility, viewing-angle dependence, etc.

However, in the TN-mode liquid crystal display devices, the liquidcrystal molecules have a refractive index anisotropy Δn, and areoriented so as to incline to the two substrates that are disposedopposite to each other. For these reasons, the viewing-angle dependenceincreases: i.e., the contrast of displayed images varies depending uponthe direction and angle of the viewer.

FIG. 17 schematically shows the cross-sectional construction of a TNliquid crystal display element 41. As a result of application of avoltage for half-tone display, liquid crystal molecules 42 shown in FIG.17 slants upward slightly. In such a liquid crystal display element 41,a linearly polarized ray 45 passing through the surfaces of a pair ofsubstrates 43 and 44 along the normals thereto, and linearly polarizedrays 46 and 47 passing through those surfaces not along the normalsthereto cross the liquid crystal molecules 42 at different angles.Besides, the liquid crystal molecules 42 have a refractive indexanisotropy Δn. Therefore, the linearly polarized rays 45, 46 and 47,upon passing through the liquid crystal molecules 42 in differentdirections, produce ordinary and extraordinary rays. The linearlypolarized rays 45, 46 and 47 are converted to elliptically polarizedrays according to the phase difference between the ordinary andextraordinary rays, which cause the viewing-angle dependence.

In addition, in an actual liquid crystal layer, the liquid crystalmolecules 42 show different tilt angles in the vicinity of the midpointbetween the substrates 43 and 44 and in the vicinities of the substrates43 and 44. The liquid crystal molecules 42 near the substrate 43 andthose near the substrate 44 are twisted by 90° about the normal.

For those reasons described so far, the linearly polarized rays 45, 46and 47 passing through the liquid crystal layer are affected by thebirefringence effect in various ways depending upon, for example, thedirection and the angle thereof, resulting in complex viewing-angledependence.

Such viewing-angle dependence can be observed, as examples, in thefollowing situations. If the viewing angle increases from the normal tothe display screen in the standard viewing direction, i.e. downward, andexceeds a certain angle, the displayed image has a distinct color(hereinafter, referred to as the coloration phenomenon), or is reversedin black and white (hereinafter, referred to as the reversionphenomenon) If the viewing angle increases from the normal in theopposite viewing direction, i.e. upward, the contrast decreasesabruptly.

The aforementioned liquid crystal display device has another problemthat the effectual range of viewing angle narrows with a larger displayscreen. When a large liquid crystal display device is viewed from ashort distance in the front thereof, the same color may appear differentin the uppermost and lowermost parts of the large screen due to theeffect of the viewing-angle dependence. This is caused by a wider rangeof viewing angle required to encompass the whole screen surface, whichis equivalent to a viewing direction which is increasingly faroff-center.

To restrain the viewing-angle dependence, Japanese Laid-Open PatentApplications No. 55-600/1980 (Tokukaisho 55-600) and No. 56-97318/1981(Tokukaisho 56-97318) suggest that a phase difference plate (phasedifference film) be inserted as an optical element having opticalanisotropy between the liquid crystal display element and one of thepolarization plates. According to the method, the elliptically polarizedray converted from a linearly polarized ray by passage through liquidcrystal molecules having refractive index anisotropy is passed throughthe phase difference plate(s) disposed on the side(s) of the liquidcrystal layer having refractive index anisotropy. Hence, the phasedifference between the ordinary and extraordinary rays are compensatedfor for all viewing angles, and the elliptically polarized ray isconverted back to the linearly polarized ray, which enables therestraint of the viewing-angle dependence.

Japanese Laid-Open Patent Application No. 5-313159/1993 (Tokukaihei5-313159), as an example, discloses a phase difference plate of theabove kind represented by a refractive index ellipsoid with one of theprincipal refractive indices parallel to the normal to the surfaces ofthe phase difference plate. Nevertheless, this phase difference platestill cannot satisfactorily restrain the reversion phenomenon thatoccurs when the viewing angle increases in the standard viewingdirection.

To solve the problem, Japanese Laid-Open Patent Application No.6-75116/1994 (Tokukaihei 6-75116, corresponding to U.S. Pat. No.5,506,706) suggests the use of a phase difference plate represented by arefractive index ellipsoid with the principal refractive indicesinclining to the normal to the surfaces of the phase difference plate.This method adopts two kinds of phase difference plates as follows.

One of the phase difference plates can be represented by such arefractive index ellipsoid that the smallest of the three principalrefractive indices is parallel to the surfaces, one of the largerprincipal refractive indices inclines to the surfaces of the phasedifference plate by an angle θ, the remaining principal refractive indexinclines to the normal to the phase difference plate by the same angleθ, and the angle θ satisfies 20°≦θ≦70°.

The other phase difference plate can be represented, in terms of therefractive index anisotropy thereof, by a refractive index ellipsoidinclining to the surfaces thereof. To be more specific, the phasedifference plate is such that the three principal refractive indicesn_(a), n_(b), and n_(c) of the refractive index ellipsoid satisfyn_(a)<n_(c)<n_(b), and that the principal refractive index n_(b) and oneof the other principal refractive indices n_(a) or n_(c)which lies inthe surface plane incline clockwise or counterclockwise about theremaining principal refractive index n_(c) or n_(a).

As the former phase difference plate, a uniaxial or biaxial phasedifference plate can be used. For the latter one, two phase differenceplates, instead of one, can be used in such a combination that the twoprincipal refractive indices n_(b) form an angle of 90°.

A liquid crystal display device, incorporating at least one such phasedifference plate between the liquid crystal display element and thepolarization plate exhibits some restraint in the contrast variations,coloration phenomenon, and reversion phenomenon caused by theviewing-angle dependence of the display screen.

However, with today's increasingly large demand on a wider effectualrange of viewing angle and superb display quality, a better restraint inthe viewing-angle dependence is crucial. In this context, the phasedifference plate disclosed in Japanese Laid-Open Patent Application No.6-75116/1994 (Tokukaihei 6-75116) above does not provide satisfactorysolutions and needs to be improved.

SUMMARY OF THE INVENTION

An object of the present invention is to offer a liquid crystal displaydevice with improved viewing angle characteristics which includes aphase difference plate represented by the aforementioned refractiveindex ellipsoid either inclining or not inclining to the surface of thephase difference plate.

In order to accomplish the object, a liquid crystal display device inaccordance with the present invention has:

-   -   a liquid crystal display element including: a pair of        light-transmitting substrates each including a transparent        electrode layer and an alignment layer on the surface thereof        facing the other; and a liquid crystal layer sandwiched by the        light-transmitting substrates and constituted by a liquid        crystal material of which the refractive index anisotropy is        specified to vary with wavelengths of rays of light within a        range that allows no viewing-angle dependent coloration to occur        on a liquid crystal screen;    -   a pair of polarizers disposed so as to sandwich the liquid        crystal display element; and    -   at least one phase difference plate disposed between the liquid        crystal display element and the pair of polarizers,    -   wherein the phase difference plate has three principal        refractive indices n_(a), n_(b), and n_(c) being mutually        related by the inequality n_(a)<n_(b)<n_(c), and the principal        refractive index n_(b) inclines to the normal to a surface of        the phase difference plate.

With the configuration, for a case where a linearly polarized ray isconverted to an elliptically polarized ray according to the phasedifference between the ordinary and extraordinary rays developed fromthe linearly polarized ray upon the passage through the liquid crystallayer possessing birefringence, a phase difference plate is used suchthat the principal refractive indices n_(a), n_(b), and n_(c) are beingmutually related by the inequality n_(a)<n_(b)<n_(c) and that theprincipal refractive index n_(b) inclines as mentioned above. Therefore,as to the phase difference plate, the aforementioned refractive indexellipsoid described by the principal refractive indices n_(a), n_(b),and n_(c) that are orthogonal to each other inclines to the surface. Thephase difference plate disposed between the liquid crystal layer and thepolarizer compensates for the phase difference between the ordinary andextraordinary rays for all viewing angles.

However, a compensation function of this kind still falls short ofsatisfying the demand for a better restraint in the viewing-angledependence.

Bearing that fact in mind, the inventors of the present invention workeddiligently and found out that the variations in the refractive indexanisotropy of the liquid crystal material in the liquid crystal layerwith wavelengths of rays of light affect the coloration on a liquidcrystal screen (display screen) substantially, which lead to thecompletion of the liquid crystal display device.

In the liquid crystal display device in accordance with the presentinvention, the refractive index anisotropy an of the liquid crystalmaterial constituting the liquid crystal layer is specified to vary withwavelengths of rays of light within a range that allows no viewing-angledependent coloration to occur on a liquid crystal screen. Thisrestrains, in the liquid crystal display device incorporating a phasedifference plate represented by a refractive index ellipsoid thatinclines to the phase difference plate, the coloration on the screenbetter. The contrast variations and reversion phenomenon are alsorestrained better than only by the compensation function by the phasedifference plate.

In order to accomplish the object, another liquid crystal display devicein accordance with the present invention has:

-   -   a liquid crystal display element including: a pair of        light-transmitting substrates each including a transparent        electrode layer and an alignment layer on the surface thereof        facing the other; and a liquid crystal layer sandwiched by the        light-transmitting substrates and constituted by a liquid        crystal material of which the refractive index anisotropy is        specified to vary with wavelengths of rays of light within a        range that allows no viewing-angle dependent coloration to occur        on a liquid crystal screen;    -   a pair of polarizers disposed so as to sandwich the liquid        crystal display element; and    -   at least one phase difference plate disposed between the liquid        crystal display element and the pair of polarizers,    -   wherein the phase difference plate has three principal        refractive indices n_(a), n_(b), and n_(c) being such that        n_(a)=n_(c)>n_(b), and the principal refractive indices n_(a)and        n_(c) being parallel to the surface of the phase difference        plate, the principal refractive index n_(b) being parallel to        the normal to the surface.

With the configuration, for a case where a linearly polarized ray isconverted to an elliptically polarized ray according to the phasedifference between the ordinary and extraordinary rays developed fromthe linearly polarized ray upon the passage through the liquid crystallayer possessing birefringence, a phase difference plate is used suchthat the principal refractive indices n_(a) and n_(c) parallel to thesurface, and the principal refractive index n_(b) parallel to the normalto the surface have the relation of n_(a)=n_(c)>n_(b). The phasedifference plate disposed between the liquid crystal layer and thepolarizer compensates for the phase difference between the ordinary andextraordinary rays for all viewing angles.

However, a compensation function of this kind still falls short ofsatisfying the demand for a better restraint in the viewing-angledependence.

Bearing that fact in mind, the inventors of the present inventionworked, along the same line as the aforementioned liquid crystal displaydevice, to complete the other liquid crystal display device based on thefindings that the variations in the refractive index anisotropy of theliquid crystal material constituting the liquid crystal layer withwavelengths of rays of light affect the coloration on a liquid crystalscreen substantially.

In the liquid crystal display device in accordance with the presentinvention, the refractive index anisotropy Δn of the liquid crystalmaterial constituting the liquid crystal layer is specified to vary withwavelengths of rays of light within a range that allows no viewing-angledependent coloration to occur on a liquid crystal screen. Thisrestrains, in the liquid crystal display device incorporating a phasedifference plate represented by a refractive index ellipsoid that doesnot incline to the phase difference plate, the coloration on the screenbetter. The contrast variations and reversion phenomenon are alsorestrained better than only by the compensation function by the phasedifference plate.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a configuration of a liquid crystaldisplay device of the first embodiment in accordance with the presentinvention, showing some components detached from others.

FIG. 2 is a an explanatory drawing showing the relation between thestandard viewing direction and the rubbing direction of an orientationfilm of a liquid crystal display device of the first and secondembodiments in accordance with the present invention.

FIG. 3 is a perspective view illustrating principal refractive indicesof a phase difference plate of the liquid crystal display device.

FIG. 4 is a perspective view illustrating an optical arrangement ofpolarization plates and phase difference plates of the liquid crystaldisplay device, showing each component separately from the others.

FIG. 5 is a graphical representation of refractive index anisotropy Δnversus wavelength of a liquid crystal material used for a liquid crystallayer of the liquid crystal display device.

FIG. 6 is a graphical representation of Δn(λ)/Δn(550) versus wavelengthof a liquid crystal material used for a liquid crystal layer of theliquid crystal display device.

FIG. 7 is a perspective view illustrating a measuring system of theviewing-angle dependence of the liquid crystal display devices of thefirst and second embodiments.

FIG. 8 is a graphical representation showing transmittance versusapplied voltage characteristics of liquid crystal of a liquid crystaldisplay device of the first example of the first embodiment.

FIG. 9 is a graphical representation showing transmittance versusapplied voltage characteristics of liquid crystal of a liquid crystaldisplay device of a comparative example for the first example.

FIG. 10 is a cross-sectional view of a configuration of a liquid crystaldisplay device of the second embodiment in accordance with the presentinvention, showing some components detached from others.

FIG. 11 is a perspective view illustrating principal refractive indicesof a phase difference plate of the liquid crystal display device of FIG.10.

FIG. 12 is a perspective view illustrating an optical arrangement ofpolarization plates and phase difference plates of the liquid crystaldisplay device of FIG. 10, showing each component separately from theothers.

FIG. 13 is a graphical representation of refractive index anisotropy Δnversus wavelength of a liquid crystal material used for a liquid crystallayer of the liquid crystal display device of FIG. 10.

FIG. 14 is a graphical representation of Δn(λ)/Δn(550) versus wavelengthof a liquid crystal material used for a liquid crystal layer of theliquid crystal display device of FIG. 10.

FIG. 15(a) is a graphical representation showing transmittance versusapplied voltage characteristics of the liquid crystal of a liquidcrystal display device of the first example of the second embodiment,when the liquid crystal display device is viewed from the right sidethereof.

FIG. 15(b) is a graphical representation showing transmittance versusapplied voltage characteristics of the liquid crystal of a liquidcrystal display device of the first example of the second embodiment,when the liquid crystal display device is viewed from the left sidethereof.

FIG. 16(a) is a graphical representation showing transmittance versusapplied voltage characteristics of the liquid crystal of a liquidcrystal display device of a comparative example for the first example ofthe second embodiment, when the liquid crystal display device is viewedfrom the right side thereof.

FIG. 16(b) is a graphical representation showing transmittance versusapplied voltage characteristics of the liquid crystal of a liquidcrystal display device of a comparative example for the first example ofthe second embodiment, when the liquid crystal display device is viewedfrom the left side thereof.

FIG. 17 is a schematic drawing illustrating the twisted orientation ofliquid crystal molecules in a TN liquid crystal display element.

DESCRIPTION OF THE EMBODIMENTS First Embodiment

Referring to FIGS. 1 through 9, the following description will discussthe first embodiment in accordance with the present invention.

As illustrated in FIG. 1, the liquid crystal display device of thepresent embodiment is provided with a liquid crystal display element 1,a pair of phase difference plates 2 and 3, and a pair of polarizationplates (polarizers) 4 and 5.

The liquid crystal display element 1 is constituted by electrodesubstrates 6 and 7 that are placed face to face with each other and aliquid crystal layer 8 that is sandwiched in between. The electrodesubstrate 6 is constructed as follows: a glass substrate (alight-transmitting substrate) 9 is provided as a base, a transparentelectrode 10, made of ITO (Indium Tin Oxide), is formed on the surface,of the glass substrate 9, facing the liquid crystal layer 8, and analignment film 11 is formed on the transparent electrode 10. Theelectrode substrate 7 is also constructed in the same manner; a glasssubstrate (a light-transmitting substrate) 12 is provided as a base, atransparent electrode 13, made of ITO, is formed on the surface, of theglass substrate 12, facing the liquid crystal layer 8, and an alignmentfilm 14 is formed on the transparent electrode 13.

Although FIG. 1 shows a construction corresponding to two pixels forconvenience of explanation, the transparent electrodes 10 and 13, whichare strips with a predetermined width, are respectively placed on theglass substrates 9 and 12 with predetermined intervals all over theliquid crystal display element 1, and are designed so that they areorthogonal to each other on the glass substrates 9 and 12, when viewedin a direction perpendicular to the substrate surfaces. Each portion atwhich the transparent electrodes 10 and 13 intersect each othercorresponds to a pixel for carrying out display, and the pixels areplaced in a matrix format over the entire structure of the presentliquid crystal display device. A voltage is applied to the transparentelectrodes 10 and 13 by a driving circuit (not shown) according todisplay data.

The electrode substrates 6 and 7 are bonded by seal resin 15, and aliquid crystal layer 8 is sealed inside the space that is formed by theelectrode substrates 6 and 7 and the seal resin 15. The liquid crystallayer 8 is made up of such a liquid crystal material that the refractiveindex anisotropy Δn thereof satisfies a predetermined condition toproduce the best properties when combined with the compensation functionof phase difference by the phase difference plates 2 and 3 (will bedescribed later in detail).

In the present liquid crystal display device, a unit, which is formed byincorporating phase difference plates 2 and 3 and polarization plates(polarizers) 4 and 5 into the above-mentioned liquid crystal displayelement 1, is referred to as a liquid crystal cell 16.

The alignment films 11 and 14 are treated with a rubbing technique inadvance so that the liquid crystal molecules between them are orientedwith a twist angle of about 90°. As shown in FIG. 2, the rubbingdirection R₁ of the alignment film 11 and the rubbing direction R₂ ofthe alignment film 14 are set to be orthogonal to each other.

The phase difference plates 2 and 3 are provided overlapping between theliquid crystal display element 1 and the polarization plate 4. The phasedifference plates 2 and 3 are constituted by a support base made up of atransparent organic high polymer and a liquid crystal polymer with apositive refractive index anisotropy provided on the support base. Theliquid crystal polymer are treated with an oblique orientation techniqueor hybrid orientation, and crosslinked. As a result, the phasedifference plates 2 and 3 are formed so as to have a refractive indexellipsoid (will be described later in detail) that slants to the phasedifference plates 2 and 3.

With respect to the support base of the phase difference plates 2 and 3,triacetylcellulose (TAC), which is generally used for polarizationplates, is suitably applied with high reliability. Besides this,colorless, transparent organic polymeric films made of polycarbonate(PC), polyethyleneterephthalate (PET), etc., which are superior inweather resistance and chemical resistance, are also suitably applied.

As illustrated in FIG. 3, each of the phase difference plates 2 and 3has principal refractive indexes n_(a), n_(b) and n_(c) pointing inthree different directions. The direction of the principal refractiveindex n_(a) coincides with the direction of the y-coordinate axis amongthe x, y, and z-coordinate axes that are orthogonal to each other. Thedirection of the principal refractive index n_(b) inclines by θ in thedirection of arrow A with respect to the z-coordinate axis (parallel toa normal to the surface) that is perpendicular to the surface of thephase difference plates 2 and 3 that corresponds to the screen.Alternatively (not shown), the principal refractive index n_(c) and thex-coordinate axis, not the principal refractive index n_(a) and they-coordinate axis, may be in the same the direction. In this case, thedirection of the principal refractive index n_(b) inclines toward oraway from the y-coordinate axis with respect to the z-coordinate axis.

The principal refractive indices n_(a), n_(b), and n_(c) of the phasedifference plates 2 and 3 are related to each other by the inequality:n_(a)<n_(b)<n_(c). Therefore, there exist two optic axes, and the phasedifference plates 2 and 3 have biaxiality and a positive refractiveindex anisotropy. The phase difference plates 2 and 3 have a firstretardation value: (n_(c)−n_(a))×d=220 nm, and a second retardationvalue: (n_(c)−n_(b))×d=35 nm, where (n_(c)−n_(a)) and (n_(c)−n_(b)) eachrepresent a refractive index anisotropy Δn, and d represents thethickness of the phase difference plates 2 and 3.

The angle θ by which the direction of the principal refractive indexn_(c) of the phase difference plates 2 and 3 inclines, i.e. theinclination angle θ of the refractive index ellipsoid, is set to anarbitrary value in the range 15°≦θ≦75°. By setting the inclination angleθ to such a value, regardless of whether the refractive index ellipsoidinclines clockwise or counterclockwise, the compensation function ofphase difference by the phase difference plates 2 and 3 is surelyachieved.

Instead of using the two phase difference plates 2 and 3, one of themmay be disposed on one side. As a further alternative, three or morephase difference plates may be used.

As illustrated in FIG. 4, the polarization plates 4 and 5 in the liquidcrystal display element 1 are arranged so that their absorption axes AX₁and AX₂ are respectively parallel to the rubbing directions R₁ and R₂ ofthe alignment films 11 and 14 (see FIG. 1). In the present liquidcrystal display device, since the rubbing directions R₁ and R₂ areorthogonal to each other, the absorption axes AX₁ and AX₂ are alsoorthogonal to each other.

Here, as illustrated in FIG. 3, a direction which is formed byprojecting the direction of the principal refractive index n_(b), whichis inclined in the direction to impart anisotropy to the phasedifference plates 2 and 3, onto the surfaces of the phase differenceplates 2 and 3 is defined as direction D.

As illustrated in FIG. 4, the phase difference plate 2 is placed so thatdirection D (direction D₁) is parallel to the rubbing direction R₂, andthe phase difference plate 3 is placed so that direction D (directionD₂) is parallel to the rubbing direction R₁.

With the above-mentioned arrangement of the phase difference plates 2and 3 and the polarization plates 4 and 5, the present liquid crystaldisplay device can carry out the so-called normally white displaywherein rays of light are allowed to pass during off-time so that whitedisplay is provided.

In general, in optical anisotropic materials such as liquid crystal andphase difference plates (phase difference films), the above-mentionedanisotropy including the three-dimensional principal refractive indexesn_(a), n_(c) and n_(b) is represented by a refractive index ellipsoidsatisfying the equation below. The refractive-index anisotropy anassumes different values depending on: which direction the refractiveindex ellipsoid is observed from. The refractive index ellipsoid for thephase difference plates 2 and 3 slants as mentioned above and isrepresented by the following equation.(x/n _(c))²+(y/n _(a))²+(z/n _(b))²=1

Next, the liquid crystal layer 8 will be explained in detail.

As mentioned above, the liquid crystal layer 8 is made up of such aliquid crystal material that the refractive index anisotropy Δn thereofsatisfies a predetermined condition to produce the best properties whencombined with the compensation function of phase difference by the phasedifference plates 2 and 3: namely, the refractive index anisotropy Δn isset in such a range that the variations in the refractive indexanisotropy Δn with wavelengths of rays do not causeviewing-angle-dependent coloration on the liquid crystal screen.

Specifically, the liquid crystal material for use is designed to meetthe following condition on specification ranges.

Δn(450)−Δn(650), i.e. the difference between the refractive indexanisotropy Δn(450) of the liquid crystal material for rays of lighthaving the wavelength of 450 nm and the refractive index anisotropyΔn(650) thereof for rays of light having the wavelength of 650 nm, isset in a range not less than 0.0070 to not more than 0.0250. Thedifference is more preferably set in a range not less than 0.0200 to notmore than 0.0250.

By using a liquid crystal material designed to meet such a condition,not only the restraint in the contrast variations, colorationphenomenon, and reversion phenomenon caused by the viewing-angledependence of the display screen by the compensation function of phasedifference by the phase difference plates 2 and 3, but the colorationphenomenon of the display screen not caused by the viewing-angledependence is also restrained.

More specifically, if a liquid crystal material designed so as to assumeat least one value in the above-mentioned wider range is used, theresultant liquid crystal display device, exhibiting coloration at theviewing angle of 50° which is typically required for liquid crystaldisplay devices, however, can display images that are up to standard forreal use for any viewing direction.

Especially, by assuming at least one value in the above-mentioned morepreferable range, the resultant liquid crystal display device is freefrom any coloration for any viewing direction at the viewing angle of70°.

Besides, by using a liquid crystal material designed to meet the range,the contrast variations and reversion phenomenon are better restrainedthan only by the compensation function by the phase difference plates 2and 3.

Moreover, in the present liquid crystal display device, the conditionbelow is preferably met as well as the aforementioned condition. Forsuch a case, more specifically, the following condition is satisfied inthe liquid crystal layer 8.

The refractive index anisotropy Δn(550) of the liquid crystal materialfor rays of light having the wavelength of 550 nm is set to be largerthan 0.060 and smaller than 0.120. More preferably, the refractive indexanisotropy Δn(550) is set to be not less than 0.070 and not more than0.095.

By meeting such an additional condition, it becomes possible to restrainnot only the viewing-angle dependence with the compensation function ofphase difference by the phase difference plates 2 and 3 and with thecompensation function based on the aforementioned range condition on thedifference between the refractive index anisotropies Δn, but also thedecrease in contrast ratio in the opposite viewing direction and thereversion phenomenon in the left and right directions.

FIG. 5 shows, in a solid curve a, Δn(λ) against wavelengths (λ), i.e.refractive index anisotropy Δn versus wavelength characteristics, of aliquid crystal material usable for the liquid crystal layer 8 of thepresent liquid crystal display device. For the purpose of comparison,FIG. 5 also shows, in an alternate long and short dash curve b, Δn(λ)against wavelengths (λ) of a liquid crystal material conventionally usedfor the liquid crystal layer of a liquid crystal display device.

It can be understood from the comparison of the curves a and b that theslope of the refractive index anisotropy an versus wavelengthcharacteristics of the present liquid crystal display device is sharperthan that of the refractive index anisotropy Δn versus wavelengthcharacteristics of a conventional liquid crystal display device.

FIG. 6 shows, in a solid curve c, Δn(λ)/Δn(550) against wavelengths (λ)of another liquid crystal material usable for the liquid crystal layer 8of the present liquid crystal display device. For the purpose ofcomparison, FIG. 6 also shows, in an alternate long and short dash curved, Δn(λ)/Δn(550) against wavelengths (λ) of a liquid crystal materialconventionally used for the liquid crystal layer of a liquid crystaldisplay device.

It can be understood from the comparison of the curves c and d that theslope depicting the changes in Δn(λ)/Δn(550) of the present liquidcrystal display device is also sharper than that depicting the changesin Δn(λ)/Δn(550) of a conventional liquid crystal display device.

The liquid crystal display device of the present embodiment configuredin this manner has a compensation function by the phase differenceplates 2 and 3 for a phase difference that occurs to the liquid crystaldisplay element 1 in accordance with the viewing angle, and acompensation function based on such a specification of the variations inthe refractive index anisotropy Δn with wavelengths of rays of lightpassing through the liquid crystal material in the liquid crystal layer8 as to fall in a range where no coloration occurs on the liquid crystalscreen. Since this properly restrains the contrast variations,coloration phenomenon, and reversion phenomenon caused by theviewing-angle dependence, images can be displayed in high quality.

Next, the following description will explain examples of the presentembodiment configured as above, together with a comparative example.

EXAMPLE 1

In the present example, five samples #1 to #5, each having a cell gap(the thickness of the liquid crystal layer 8) set to 5 82 m, were usedas samples of the liquid crystal display device shown in FIG. 1.Δn(450)−Δn(650), i.e. the difference between the refractive indexanisotropy Δn(450) of the liquid crystal material constituting theliquid crystal layer 8 for the wavelength of 450 nm and the refractiveindex anisotropy Δn(650) thereof for the wavelength of 650 nm, was setto 0.0070, 0.0090, 0.0120, 0.0200 and 0.0250 for the samples #1 to #5respectively.

The phase difference plates 2 and 3 of the samples #1 to #5 were eachconstituted by a transparent support base (e.g. triacetylcellulose(TAC)) and a liquid crystal polymer with a positive refractive indexanisotropy that was provided on the support base, treated with anoblique orientation technique, and crosslinked. The first and secondretardation values of the phase difference plates 2 and 3 wererespectively set to 220 nm and 35 nm as mentioned above. Since theinclination angle θ of the refractive index ellipsoid for the phasedifference plates 2 and 3 was 20°, the direction of the principalrefractive index n_(b) inclined by about 20° in the direction of arrow Awith respect to the z-coordinate axis among the x, y, and z-coordinateaxes, and the direction of the principal refractive index n_(c) inclinedby about 20° in the direction of arrow B with respect to thex-coordinate axis.

A comparative sample #100 was prepared as a comparative example for thepresent example, the comparative sample #100 having the sameconfiguration as the present example except that a liquid crystalmaterial of Δn(450)−Δn(650)=0.0045 was used for the liquid crystal layer8 of the liquid crystal display device shown in FIG. 1.

Table 1 shows results of visual inspections of the samples #1 to #5 andthe comparative sample #100 in white light.

TABLE 1 Δn(450) − Δn(650) (× 10⁻³) Viewing Angles 4.5 7.0 9.0 12.0 20.025.0 (θ) #100 #1 #2 #3 #4 #5 50° NG G E E E E 60° NG NG G E E E 70° NGNG NG G E E

In Table 1, E stands for “Excellent” and indicates that no colorationwas observed, G stands for “Good” and indicates that coloration wasobserved to the extent that did not pose any problem for real use, andNG stands for “No Good” and indicates that coloration was so evident asto be intolerable for real use.

The samples #4 and #5 of the example produced good image quality with nocoloration observed at all from any viewing direction at the viewingangle of 70°. The sample #3 produced good image quality with nocoloration observed at all from any viewing direction up to the viewingangle of 60°. The sample #2 produced good image quality with nocoloration observed at all from any viewing direction up to the viewingangle of 50°. The sample #1 exhibited coloration when viewed from theleft and right directions at the viewing angle of 50°, but thecoloration was only to an extent that was tolerable for real use.

By contrast, the comparative sample #100 exhibited yellow-to-orangecoloration to the extent that was intolerable for real use when viewedfrom the left and right directions at the viewing angle of 50°.

The same results were obtained from the samples having such phasedifference plates 2 and 3 that the liquid crystal polymers provided onthe transparent support base were treated with a hybrid orientationtechnique.

EXAMPLE 2

As illustrated in FIG. 7, the measuring system on the viewing-angledependence of the liquid crystal display device is provided with alight-receiving element 21, an amplifier 22 and a recording device 23.The liquid crystal cell 16 is placed so that the surface 16 a thereoffacing the glass substrate 9 is located on the reference surface x-y ofthe rectangular coordinates x, y and z. The light-receiving element 21is an element capable of receiving light with a constant stereoscopiclight-receiving angle, and is placed at a position a predetermineddistance apart from the coordinate origin in a direction making an angleφ (viewing angle) with respect to the z-direction that is perpendicularto the surface 16 a.

During the measuring process, monochromatic light with a wavelength of550 nm is directed to the liquid crystal cell 16 installed in thepresent measuring system through a surface of the liquid crystal cell 16that is opposite to the surface 16 a. One part of the monochromaticlight that has passed through the liquid crystal cell 16 is madeincident on the light-receiving element 21. The output of thelight-receiving element 21, after having been amplified to apredetermined level by the amplifier 22, is recorded by the recordingdevice 23 such as a waveform memory or a recorder.

In the present example, three samples #6 to #8, each having a cell gapset to 5 μm, were used. The refractive index anisotropy Δn(550) of theliquid crystal material constituting the liquid crystal layer 8 in theliquid crystal cell 16 shown in FIG. 1 for the wavelength of 550 nm wereset to 0.070, 0.080 and 0.095 for the samples #6 to #8 respectively. Thephase difference plates 2 and 3 of the samples #6 to #8 were the same asthose of the aforementioned first example in which the liquid crystalpolymer was treated with an oblique orientation technique.

These samples #6 to #8 were placed in the measuring system shown in FIG.7 to measure the output levels of the light receiving element 21 fixedat a constant angle φ in response to voltages applied across the samples#6 to #8.

The measurement was done with the light receiving element 21 disposed sothat the angle φ equaled 50° and fixed upward (in the opposite viewingdirection), on the presumption that the y direction is toward the leftside of the display screen and the x direction is toward the bottom sideof the display screen.

The results of the measurement are plotted in the graph of FIG. 8 astransmittances of light against voltages applied across the samples #6to #8 (transmittance versus applied voltage characteristics). In FIG. 8,the alternate long and short dash curve L1 represents thecharacteristics of the sample #6, the solid curve L2 represents thecharacteristics of the sample #7, and the broken curve L3 represents thecharacteristics of the sample #8.

Two comparative samples #101 and #102 were prepared as a comparativeexample for the present example, the comparative samples #101 and #102having the same configurations as the samples #6 to #8 except thatliquid crystal materials having refractive index anisotropies Δn(550)for the wavelength of 550 nm set to 0.060 and 0.120 respectively wereused for the liquid crystal layer 8 of the liquid crystal cell 16 shownin FIG. 1. In the same manner as the present example, these comparativesamples #101 and #102 were placed in the measuring system shown in FIG.7 to measure the output levels of the light receiving element 21 fixedat a constant angle φ in response to voltages applied across thecomparative samples #101 and #102.

The measurement was done with the light receiving element 21 disposed sothat the angle φ equaled 50° and fixed upward.

The results of the measurement are plotted in the graph of FIG. 9 astransmittances of light against voltages applied across the comparativesamples #101 and #102 (transmittance versus applied voltagecharacteristics). In FIG. 9, the solid curve L10 represents thecharacteristics of the comparative sample #101, and the broken curve L11represents the characteristics of the comparative sample #102.

The samples #6 to #8 of the present example were compared with thecomparative samples #101 and #102 of the comparative example withrespect to the upward transmittance versus applied voltagecharacteristics. As illustrated by L1 to L3 in FIG. 8, the transmittanceof the samples #6 to #8 decreased by substantial amounts with highervoltages applied across the samples #6 to #8. By contrast, asillustrated by L11 in FIG. 9, the transmittance of the comparativesample #102 decreased only by small amounts with higher voltages appliedacross the comparative sample #102. As illustrated by L10 in FIG. 9, thetransmittance of the comparative sample #101 exhibited the reversionphenomenon, increasing with higher voltages applied across thecomparative sample #101 after the initial drop.

The samples #6 and #8 produced good image quality with no colorationobserved at all from any viewing direction at the viewing angle of 50°.By contrast, the comparative samples #101 and #102 exhibitedyellow-to-orange coloration when viewed from the left and rightdirections at the viewing angle of 50°.

The characteristics shown in FIG. 8 clearly tell that the viewing anglewidened when liquid crystal materials having refractive indexanisotropies Δn(550) for the wavelength of 550 nm set to 0.070, 0.080and 0.095 respectively were used for the liquid crystal layer 8 asmentioned above, because the transmittance decreased by substantialamounts with higher voltages and no reversion phenomenon occurred. It isunderstood that in this case the display quality of the liquid crystaldisplay device improved remarkably with no coloration taking place.

On the other hand, the characteristics shown in FIG. 9 tell that theviewing-angle dependence is not restrained sufficiently when liquidcrystal materials having refractive index anisotropies Δn(550) set to0.060 and 0.120 respectively were used for the liquid crystal layer 8.

The same results as from the samples #6 to #8 were obtained from sampleshaving the same configurations as the samples #6 to #8 except that theliquid crystal polymers constituting the phase difference plates 2 and 3were treated with a hybrid orientation technique. Similarly, the sameresults as from the comparative samples #101 and #102 were obtained fromcomparative samples having the same configurations as the comparativesamples #101 and #102 except that the liquid crystal polymersconstituting the phase difference plates 2 and 3 were treated with ahybrid orientation technique.

The dependence of the transmittance versus applied voltagecharacteristics of liquid crystal on the inclination angle θ wasexamined by changing the inclination angle θ of the refractive indexellipsoid for the phase difference plates 2 and 3; it turned out thatwhen the inclination angle θ was such that 15°≦θ≦75°, the dependence didnot basically alter regardless of the orientation state of the liquidcrystal polymers of the phase difference plates 2 and 3. Note that itwas found out that when the inclination angle θ was outside the range,the viewing angle in the opposite viewing direction did not widen.

Based on the results of visual inspections of the comparative samples#101 and #102, three comparative samples #9 to #11 were furtherprepared, the comparative samples #9 to #11 having the sameconfigurations as the samples #6 to #8 except that liquid crystalmaterials having refractive index anisotropies Δn(550) of 0.065, 0.100and 0.115 respectively were used for the liquid crystal layer 8. Thesesamples #9 to #11 were also placed in the measuring system shown in FIG.7 to measure the output levels of the light receiving element 21 inresponse to voltages applied across the samples #9 to #11. The visualinspections were conducted of the samples #9 to #11 in white light.

The samples #10 and #11 produced good upward transmittance when theangle φ equaled 50°. By contrast, the sample #9 exhibited upwardtransmittance that initially decreased to a minimum value and thenincreased with higher voltages in a similar, however, more moderatemanner than the comparative sample #101 (see FIG. 9). Therefore, thesample #9, although not producing as good transmittance as the samples#10 and #11, still could produce transmittance that was tolerable forreal use.

In the visual inspections, the samples #10 and #11 exhibitedyellow-to-orange coloration to the extent that did not pose any problemfor real use. The sample #9 exhibited bluish coloration only to a smallextent that did not pose any problem for real use.

As a supplement, a voltage of about 1 V was applied across the sample #9and the comparative sample #101 to measure the transmittance in thenormal direction to the surface of the liquid crystal cell 16 duringwhite display. The comparative sample #101 exhibited a decrease intransmittance to the extent that was intolerable for real use, while thesample #9 exhibited a decrease in transmittance to an extent that wastolerable for real use.

As described so far, a liquid crystal display device having a basicconfiguration in accordance with the present embodiment includes:

-   -   a liquid crystal display element 1 including: a pair of glass        substrates 9 and 12 including on the surfaces thereof facing        each other transparent electrodes 10 and 13 and alignment films        11 and 14; and a liquid crystal layer 8 sandwiched between the        glass substrates 9 and 12;    -   a pair of polarization plates 4 and 5 disposed so as to sandwich        the liquid crystal display element 1; and    -   at least one phase difference plate 2 (2 and 3) disposed between        the liquid crystal display element 1 and the polarization plate        4 or 5, and having three principal refractive indices n_(a),        n_(b), and n_(c) pointing in three different directions that are        orthogonal to each other, the principal refractive indices        n_(a), n_(b), and n_(c) being mutually related by the inequality        n_(a)<n_(b)<n_(c), the principal refractive index n_(b)        inclining to the normal to the surface of the phase difference        plate 2, the principal refractive index n_(b) and one of the        other principal refractive indices n_(a) and n_(c) which is not        parallel to the surface inclining clockwise or counterclockwise        about the remaining principal refractive index n_(c) or n_(a)        which is parallel to the surface,    -   wherein the liquid crystal layer 8 is constituted by a liquid        crystal material of which the refractive index anisotropy Δn is        specified to vary with wavelengths of rays of light within a        range that allows no viewing-angle dependent coloration to occur        on the liquid crystal screen.

This restrains, in the liquid crystal display device, the phasedifference of the liquid crystal display element 1 better than does thecompensation function by the phase difference plates 2 and 3 alone. Theviewing-angle dependent coloration of the liquid crystal screen isespecially restrained better. Consequently, such a liquid crystaldisplay device, including the phase difference plates 2 and 3 and theliquid crystal display element 1 can restrain the reversion phenomenon,the decrease in contrast ratio in the opposite viewing direction, andthe coloration phenomenon.

The aforementioned range is, more specifically, such thatΔn(450)−Δn(650), i.e. the difference between the refractive indexanisotropy Δn(450) of the liquid crystal material for rays of lighthaving the wavelength of 450 nm and the refractive index anisotropyΔn(650) thereof for rays of light having the wavelength of 650 nm, isnot less than 0.0070 and not more than 0.0250. A more preferred range issuch that Δn(450)−Δn(650) is not less than 0.0200 and not more than0.0250.

Especially, by specifying Δn(450)−Δn(650) to be not less than 0.0070 andnot more than 0.0250, the resultant liquid crystal display device,exhibiting coloration at the viewing angle of 50° which is typicallyrequired for liquid crystal display devices, however, achieves wellrestrained coloration to the extent that is up to standard for real usefor any viewing direction.

Moreover, by specifying Δn(450)−Δn(650) to be not less than 0.0200 andnot more than 0.0250, the resultant liquid crystal display device cancarry out display that is totally free from the coloration phenomenonfor any viewing direction at the viewing angle of 70° which is typicallyrequired for wide viewing-angle liquid crystal display devices.

As described here, the above-mentioned configuration can remarkablyimprove the quality of the images displayed by the liquid crystaldisplay device, since the contrast ratio in black-and-white display isnot affected by the viewing direction of a viewer.

Besides, in a liquid crystal display device having the aforementionedbasic configuration and such a liquid crystal display device thatΔn(450)−Δn(650) is set to be not less than 0.0070 and not more than0.0250, since the refractive index anisotropy Δn(550) of the liquidcrystal material for rays of light having the wavelength of 550 nm isset to be larger than 0.060 and smaller than 0.120, the phase differencethat occurs to the liquid crystal display element 1 according to theviewing angle is eliminated. This is based on the aforementionedobservations of decreases in the reversion phenomenon and contrast ratiofor some viewing directions when the refractive index anisotropy Δn(550)of the liquid crystal material for rays of light having the wavelengthof 550 nm that is the central range of the visible region of spectrum isset to be not more than 0.060 or not less than 0.120. Therefore, thecontrast variations and reversion phenomenon in the left and rightdirections, not to mention the coloration phenomenon caused by theviewing-angle dependence, can be further restrained on the displayscreen.

With the liquid crystal display device thus configured, if therefractive index anisotropy Δn(550) of the liquid crystal material forrays of light having the wavelength of 550 nm is set to be not less than0.070 and not more than 0.095, the contrast variations caused by theviewing-angle dependence and reversion phenomenon in the left and rightdirections can be even further restrained.

Among the liquid crystal display devices incorporating theaforementioned basic configuration, in such a liquid crystal displaydevice that Δn(450)−Δn(650) is set to be not less than 0.0070 and notmore than 0.0250 and such a liquid crystal display device that Δn(550)is set to be not less than 0.060 and not more than 0.120, since theinclination angle of the refractive index ellipsoid for all the phasedifference plates is set in the range 15° to 75°, it becomes possible toensure the compensation function of phase difference by the phasedifference plates 2 and 3. Consequently, the visibility can be surelyimproved.

Second Embodiment

Referring to FIGS. 2, 7, and 10 through 16, the following descriptionwill discuss the second embodiment in accordance with the presentinvention. Here, for convenience, members of the second embodiment thathave the same function as members of the first embodiment, and that arementioned in the first embodiment are indicated by the same referencenumerals and description thereof is omitted.

As illustrated in FIG. 10, the liquid crystal display device of thepresent embodiment is provided with a liquid crystal display element 31,a pair of phase difference plates 32 and 33, and a pair of polarizationplates 4 and 5.

The liquid crystal display element 31 has the same configuration as theliquid crystal display element 1. However, a liquid crystal layer 34made up of a liquid crystal material different from that for the liquidcrystal layer 8 is sandwiched between the electrode substrates 6 and 7that are placed face to face with each other. The liquid crystalmaterial constituting the liquid crystal layer 34 is such that therefractive index anisotropy Δn thereof satisfies a predeterminedcondition to produce the best properties when combined with thecompensation function of phase difference by the phase difference plates32 and 33.

In the present liquid crystal display device, a unit, which is formed byincorporating the phase difference plates 32 and 33 and the polarizationplates (polarizers) 4 and 5 into the above-mentioned liquid crystaldisplay element 1, is referred to as a liquid crystal cell 35.

The phase difference plates 32 and 33 are provided between the liquidcrystal display element 31 and the respective polarization plates 4 and5 that are disposed so as to sandwich the liquid crystal display element31. The phase difference plates 32 and 33 are constituted by a supportbase made up of a transparent organic high polymer and discotic liquidcrystal provided on the support base. The discotic liquid crystal istreated with a horizontal orientation technique and crosslinked. Thesupport base of the phase difference plates 32 and 33 is suitably madeup of the same material as that for the phase difference plates 2 and 3.

As illustrated in FIG. 11, each of the phase difference plates 32 and 33has principal refractive indexes n_(a), n_(b) and n_(c) pointing inthree different directions. The direction of the principal refractiveindex n_(a) coincides with the direction of the y-coordinate axis amongthe x, y, and z-coordinate axes that are orthogonal to each other, whilethe direction of the principal refractive index n_(c) coincides with thedirection of the x-coordinate axis. The direction of the principalrefractive index n_(b) coincides with the direction of the z-coordinateaxis (the direction normal to the surface) which is perpendicular to thesurface corresponding to the display screen of the phase differenceplates 32 and 33. That is, the refractive index ellipsoid does notincline with respect to the phase difference plates 32 and 33.

The principal refractive indices n_(a), n_(b), and n_(c) of the phasedifference plates 32 and 33 are such that n_(a)=n_(c)>n_(b). Therefore,there exists only one optic axis, and the phase difference plates 32 and33 have uniaxiality and a negative refractive index anisotropy. Sincen_(a)=n_(c), the phase difference plates 32 and 33 have a firstretardation value (n_(c)−n_(a))×d almost equal to 0 nm, and a secondretardation value (n_(c)−n_(b))×d specified arbitrarily in the rangefrom 80 nm to 250 nm, where (n_(c)−n_(a)) and (n_(c)−n_(b)) eachrepresent a refractive index anisotropy Δn, and d represents thethickness of the phase difference plates 32 and 33. By specifying thesecond retardation value (n_(c)−n_(b))×d in that range, the compensationfunction of phase difference by the phase difference plates 32 and 33 issurely achieved.

Instead of using the two phase difference plates 32 and 33, one of themmay be disposed on one side, or both of them are stacked together anddisposed on one side. As a further alternative, three or more phasedifference plates may be used.

As illustrated in FIG. 12, in the present liquid crystal display device,the polarization plates 4 and 5 in the liquid crystal display element 31are arranged so that their absorption axes AX₁ and AX₂ are respectivelyparallel to the rubbing directions R₁ and R₂ of the alignment films 11and 14 (see FIG. 1). In the present liquid crystal display device, sincethe rubbing directions R₁ and R₂ are orthogonal to each other, theabsorption axes AX₁ and AX₂ are also orthogonal to each other.

The phase difference plate 32 is placed so that direction D₃ of theprincipal refractive index n_(c) thereof is parallel to the rubbingdirection R₁, the phase difference plate 33 is placed so that directionD₄ of the principal refractive index n_(c) thereof is parallel to therubbing direction R₂.

With the above-mentioned arrangement of the phase difference plates 32and 33 and the polarization plates 4 and 5, the present liquid crystaldisplay device can carry out the so-called normally white displaywherein rays of light are allowed to pass during off-time so that whitedisplay is provided.

Next, the liquid crystal layer 34 will be explained in detail.

As mentioned above, the liquid crystal layer 34 is made up of such aliquid crystal material that the refractive index anisotropy Δn thereofsatisfies a predetermined condition to produce the best properties whencombined with the compensation function of phase difference by the phasedifference plates 32 and 33: namely, the refractive index anisotropy anis set in such a range that the variation in the refractive indexanisotropy Δn with wavelengths of rays do not causeviewing-angle-dependent coloration on the liquid crystal screen.

Specifically, the liquid crystal material for use is designed to meetthe following condition on specification ranges.

Δn(450)−Δn(650), i.e. the difference between the refractive indexanisotropy Δn(450) of the liquid crystal material for rays of lighthaving the wavelength of 450 nm and the refractive index anisotropyΔn(650) thereof for rays of light having the wavelength of 650 nm, isset in a range not less than 0 to less than 0.0090. The difference ismore preferably set in a range not less than 0 to not more than 0.0045.

By using a liquid crystal material designed to meet such a condition,not only the restraint in the contrast variations, colorationphenomenon, and reversion phenomenon caused by the viewing-angledependence of the display screen by the compensation function of phasedifference by the phase difference plates 32 and 33, but the colorationphenomenon of the display screen not caused by the viewing-angledependence is also restrained.

More specifically, if a liquid crystal material designed so as tosatisfy the above-mentioned wider range is used, the resultant liquidcrystal display device, exhibiting coloration at the viewing angle of50° which is typically required for liquid crystal display devices,however, can display images that are up to standard for real use for anyviewing direction.

Especially, by satisfying the above-mentioned more preferable range, theresultant liquid crystal display device is free from any coloration forany viewing direction at the viewing angle of 70°.

Besides, by using a liquid crystal material designed to meet thecondition, the contrast variations and reversion phenomenon are betterrestrained than only by the compensation function by the phasedifference plates 32 and 33.

Moreover, in the present liquid crystal display device, the conditionbelow is preferably met as well as the aforementioned condition. Forsuch a case, more specifically, the following condition is satisfied inthe liquid crystal layer 34.

The refractive index anisotropy Δn(550) of the liquid crystal materialfor rays of light having the wavelength of 550 nm should be set to belarger than 0.060 and smaller than 0.120. More preferably, therefractive index anisotropy Δn(550) should be set to be not less than0.070 and not more than 0.095.

By meeting such an additional condition, it becomes possible to restrainnot only the viewing-angle dependence with the compensation function ofphase difference by the phase difference plates 32 and 33 and with thecompensation function based on the aforementioned range condition on thedifference between the refractive index anisotropies Δn, but also thedecrease in contrast ratio in the opposite viewing direction and thereversion phenomenon in the left and right directions.

FIG. 13 shows, in a solid curve e, Δn(λ) against wavelengths (λ), i.e.refractive index anisotropy Δn versus wavelength characteristics, of aliquid crystal material usable for the liquid crystal layer 34 of thepresent liquid crystal display device. For the purpose of comparison,FIG. 13 also shows, in an alternate long and short dash curve f, Δn(λ)against wavelengths (λ) of a liquid crystal material conventionally usedfor the liquid crystal layer of a liquid crystal display device.

The curve e shows an almost flat overall profile, falling slightly withhigher wavelengths (λ). It can be understood from the comparison of thecurves e and f that the slope of the refractive index anisotropy Δnversus wavelength characteristics of the present liquid crystal displaydevice is less sharp than that of the refractive index anisotropy Δnversus wavelength characteristics of a conventional liquid crystaldisplay device.

FIG. 14 shows, in a solid curve g, Δn(λ)/Δn(550) against wavelengths (λ)of another liquid crystal material usable for the liquid crystal layer34 of the present liquid crystal display device. For the purpose ofcomparison, FIG. 14 also shows, in an alternate long and short dashcurve h, Δn(λ)/Δn(550) against wavelengths (λ) of a liquid crystalmaterial conventionally used for the liquid crystal layer of a liquidcrystal display device.

It can be understood from the comparison of the curves g and h that theslope depicting the changes in Δn(λ)/Δn(550) of the present liquidcrystal display device is also less sharp than that depicting thechanges in Δn(λ)/Δn(550) of a conventional liquid crystal displaydevice.

The liquid crystal display device of the present embodiment configuredin this manner has a compensation function by the phase differenceplates 32 and 33 for a phase difference that occurs to the liquidcrystal display element 31 in accordance with the viewing angle, and acompensation function based on such a specification of the variations inthe refractive index anisotropy an with wavelengths of rays of lightpassing through the liquid crystal material in the liquid crystal layer34 as to fall in a range where no coloration occurs on the liquidcrystal screen. Since this properly restrains the contrast variations,coloration phenomenon, and reversion phenomenon caused by theviewing-angle dependence, images can be displayed in high quality.

Next, the following description will explain examples of the presentembodiment configured as above, together with a comparative example.

EXAMPLE 1

In the present example, five samples #21 to #25, each having a cell gap(the thickness of the liquid crystal layer 34) set to 5 μm, were used assamples of the liquid crystal display device shown in FIG. 10.Δn(450)−Δn(650), i.e. the difference between the refractive indexanisotropy Δn(450) of the liquid crystal material constituting theliquid crystal layer 34 for the wavelength of 450 nm and the refractiveindex anisotropy Δn(650) thereof for the wavelength of 650 nm, was setto 0, 0.0030, 0.0045, 0.0055, and 0.0070 for the samples #21 to #25respectively.

The phase difference plates 32 and 33 of the samples #21 to #25 wereeach constituted by a transparent support base (e.g. triacetylcellulose(TAC)) and discotic liquid crystal that was provided on the supportbase, treated with a horizontal orientation technique, and crosslinked.The first and second retardation values of the phase difference plates32 and 33 were respectively set to 0 nm and 100 nm.

A comparative sample #200 was prepared as a comparative example for thepresent example, the comparative sample #200 having the sameconfiguration as the present example except that a liquid crystalmaterial of Δn(450)−Δn(650)=0.0090 was used for the liquid crystal layer34 of the liquid crystal display device shown in FIG. 10.

Table 2 shows results of visual inspections of the samples #21 to #25and the comparative sample #200 in white light.

TABLE 2 Δn(450) − Δn(650) (× 10⁻³) Viewing Angles 0 3.0 4.5 5.5 7.0 9.0(θ) #21 #22 #23 #24 #25 #200 50° E E E E G NG 60° E E E G NG NG 70° E EE NG NG NG

In Table 2, E stands for “Excellent” and indicates that no colorationwas observed, G stands for “Good” and indicates that coloration wasobserved to the extent that did not pose any problem for real use, andNG stands for “No Good” and indicates that coloration was so evident asto be intolerable for real use.

The samples #21 to #23 of the example produced good image quality withno coloration observed at all from any viewing direction at the viewingangle of 70°. The sample #24 produced good image quality with nocoloration observed at all from any viewing direction at the viewingangle of 50°, and exhibited a little coloration when viewed from theleft and right directions at the viewing angle of 60°, but thecoloration was only to an extent that was tolerable for real use. Thesample #25 also exhibited a little coloration when viewed from the leftand right directions at the viewing angle of 50°, but the coloration wasalso only to an extent that was tolerable for real use.

By contrast, the comparative sample #200 exhibited yellow-to-orangecoloration to the extent that was intolerable for real use when viewedfrom the left and right directions even at the viewing angle of 50°.

EXAMPLE 2

Similarly to the second example of the first embodiment, the measuringsystem illustrated in FIG. 7 was used to measure the viewing-angledependence of the liquid crystal display device. The liquid crystal cell35 of the liquid crystal display device is placed so that the surface 35a thereof facing the glass substrate 9 is located on the referencesurface x-y of the rectangular coordinates x, y and z.

During the measuring process, monochromatic light with a wavelength of550 nm is directed to the liquid crystal cell 35 installed in thepresent measuring system through a surface of the liquid crystal cell 35that is opposite to the surface 35 a. One part of the monochromaticlight that has passed through the liquid crystal cell 35 is madeincident on the light-receiving element 21. The output of thelight-receiving element 21, after having been amplified to apredetermined level by the amplifier 22, is recorded by the recordingdevice 23 such as a waveform memory or a recorder.

In the present example, three samples #26 to #28, each having a cell gapset to 5 μm, were used. The refractive index anisotropy Δn(550) of theliquid crystal material constituting the liquid crystal layer 34 in theliquid crystal cell 35 shown in FIG. 10 for the wavelength of 550 nmwere set to 0.070, 0.080 and 0.095 for the samples #26 to #28respectively. The phase difference plates 32 and 33 of the samples #26to #28 were the same as those of the aforementioned first example inwhich the discotic liquid crystal was treated with a horizontalorientation technique.

These samples #26 to #28 were placed in the measuring system shown inFIG. 7 to measure the output levels of the light receiving element 21fixed at a constant angle φ in response to voltages applied across thesamples #26 to #28.

The measurement was done with the light receiving element 21 disposed sothat the angle φ equaled 50° and moved between the right and leftdirections, on the presumption that the y direction is toward the leftside of the display screen and the x direction is toward the bottom sideof the display screen.

The results of the measurement are plotted in the graphs of FIGS. 15(a)and 15(b) as transmittances of light against voltages applied across thesamples #26 to #28 (transmittance versus applied voltagecharacteristics), FIG. 15(a) showing the results of the measurement donefrom the right direction of FIG. 2, and FIG. 15(b) showing the resultsof the measurement done from the left direction of FIG. 2.

In FIGS. 15(a) and 15(b), the alternate long and short dash curves L21and L24 represent the characteristics of the sample #26, the solidcurves L22 and L25 represent the characteristics of the sample #27, andthe broken curves L23 and L26 represent the characteristics of thesample #28.

Two comparative samples #201 and #202 were prepared as a comparativeexample for the present example, the comparative samples #201 and #202having the same configurations as the samples #26 to #28 except thatliquid crystal materials having refractive index anisotropies Δn(550)for the wavelength of 550 nm set to 0.060 and 0.120 respectively wereused for the liquid crystal layer 34 of the liquid crystal cell 35 shownin FIG. 10. In the same manner as the present example, these comparativesamples #201 and #202 were placed in the measuring system shown in FIG.7 to measure the output levels of the light receiving element 21 fixedat a constant angle φ in response to voltages applied across thecomparative samples #201 and #202.

The measurement was done with the light receiving element 21 disposed sothat the angle φ equaled 50° and moved between the right and leftdirections.

The results of the measurement are plotted in the graphs of FIGS. 16(a)and 16(b) as transmittances of light against voltages applied across thecomparative samples #201 and #202 (transmittance versus applied voltagecharacteristics), FIG. 16(a) showing the results of the measurement donefrom the right direction of FIG. 2, and FIG. 16(b) showing the resultsof the measurement done from the left direction of FIG. 2.

In FIGS. 16(a) and 16(b), the solid curves L30 and L32 represent thecharacteristics of the comparative sample #201, and the broken curvesL31 and L33 represent the characteristics of the comparative sample#202.

The samples #26 to #28 of the present example were compared with thecomparative samples #201 and #202 of the comparative example withrespect to the right-direction transmittance versus applied voltagecharacteristics. As illustrated by the curves L21 to L23 in FIG. 15(a),the transmittance of the samples #26 to #28 decreased almost to zerowith higher voltages applied across the samples #26 to #28. Asillustrated by the curve L30 in FIG. 16(a), similarly to FIG. 15(a), thetransmittance of the comparative sample #201 decreased almost to zerowith higher voltages applied across the comparative sample #201. Bycontrast, as illustrated by L31, the transmittance of the comparativesample #202 exhibited the reversion phenomenon, increasing with highervoltages applied across the comparative sample #202 after the initialdrop.

Similar results were obtained from the comparison of the samples #26 to#28 and the comparative sample #201 with respect to the left-directiontransmittance versus applied voltage characteristics. As illustrated bythe curves L24 to L26 in FIG. 15(b) and the curve L32 in FIG. 16(b), thetransmittance of the samples #26 to #28 decreased almost to zero withhigher voltages applied across the samples #26 to #28 and thecomparative sample #201. By contrast, as illustrated by L33 in FIG.16(b), the transmittance of the comparative sample #202 exhibited thereversion phenomenon, increasing with higher voltages applied across thecomparative sample #202 after the initial drop.

Visual inspections were conducted for the samples #26 to #28 and thecomparative samples #201 and #202 in white light.

The samples #26 and #28 of the present example produced goodtransmittance with no coloration observed at all from any viewingdirection at the angle φ equal to 50°. By contrast, the comparativesamples #201 and #202 exhibited yellow-to-orange coloration when viewedfrom the left and right directions at the angle φ equal to 50°.

The characteristics shown in FIGS. 15(a) and 15(b) clearly tell that theviewing angle widened when liquid crystal materials having refractiveindex anisotropies Δn(550) for the wavelength of 550 nm set to 0.070,0.080 and 0.095 respectively were used for the liquid crystal layer 34as mentioned above, because the transmittance decreased by substantialamounts with higher voltages and no reversion phenomenon occurred. Thespecifications eliminate coloration too. Therefore it is possible toimprove the display quality remarkably with the present liquid crystaldisplay device, compared with conventional liquid crystal displaydevices.

On the other hand, the characteristics shown in FIGS. 16(a) and 16(b)tell that the viewing-angle dependence is not restrained sufficientlywhen liquid crystal materials having refractive index anisotropiesΔn(550) for the wavelength of 550 nm set to 0.060 and 0.120 respectivelywere used for the liquid crystal layer 34.

Using the samples #26 to #28 of the present example, the dependence ofthe transmittance versus applied voltage characteristics of liquidcrystal on the second retardation value was examined by changing thesecond retardation value of the phase difference plates 32 and 33; itturned out that when the second retardation value was in a range of 80nm to 250 nm, the transmittance versus applied voltage characteristicsremained almost the same. By contrast, when the second retardation valuewas outside the range, the viewing angle in the horizontal direction(left-to-right direction) did not widen.

Based on the results of visual inspections of the comparative samples#201 and #202, three comparative samples #29 to #31 were furtherprepared, the comparative samples #29 to #31 having the sameconfigurations as the samples #26 to #28 except that liquid crystalmaterials having refractive index anisotropies Δn(550) for thewavelength of 550 nm of 0.065, 0.100 and 0.115 respectively were usedfor the liquid crystal layer 34. These samples #29 to #31 were alsoplaced in the measuring system shown in FIG. 7 to measure the outputlevels of the light receiving element 21 in response to voltages appliedacross the samples #29 to #31. The visual inspections were conducted ofthe samples #29 to #31 in white light.

The sample #30 (Δn(550)=0.100) and the sample #31 (Δn(550)=0.115)exhibited a slight increase in transmittance in the right and leftdirections with higher applied voltages when the angle φ equaled 50°.However, no reversion phenomenon was visually observed, and the increasein transmittance occurred only to an extent that was tolerable for realuse. Meanwhile, no problem was found with the sample #29 (Δn(550)=0.065)in the right and left directions.

In the visual inspections, the samples #30 and #31 exhibitedyellow-to-orange coloration to the extent that did not pose any problemfor real use. The sample #29 exhibited bluish coloration only to a smallextent that did not pose any problem for real use.

As a supplement, a voltage of about 1 V was applied across the sample#29 and the comparative sample #201 to measure the transmittance in thenormal direction to the surface of the liquid crystal cell 35 duringwhite display. The comparative sample #201 exhibited a decrease intransmittance to the extent that was intolerable for real use, while thesample #29 exhibited a slight decrease in transmittance to an extentthat was tolerable for real use.

As described so far, a liquid crystal display device in accordance withthe present embodiment includes:

-   -   a liquid crystal display element 31 including: a pair of glass        substrates 9 and 12 including on the surfaces thereof facing        each other transparent electrodes 10 and 13 and alignment films        11 and 14; and a liquid crystal layer 34 sandwiched between the        glass substrates 9 and 12;    -   a pair of polarization plates 4 and 5 disposed so as to sandwich        the liquid crystal display element 31; and    -   phase difference plates 32 and 33 disposed between the liquid        crystal display element 31 and the polarization plates 4 and S        disposed so as to sandwich the liquid crystal display element        31, and having three principal refractive indices n_(a), n_(b),        and n_(c) being such that n_(a)=n_(c)>n_(b), the principal        refractive indices n_(a) and n_(c) being parallel to the        surfaces of the phase difference plates 32 and 33, the principal        refractive index n_(b) being parallel to the normal to the        surfaces,    -   wherein the liquid crystal layer 34 is constituted by a liquid        crystal material of which the refractive index anisotropy Δn is        specified to vary with wavelengths of rays of light within a        range that allows no viewing-angle dependent coloration to occur        on the liquid crystal screen.

This restrains, in the liquid crystal display device, the phasedifference of the liquid crystal display element 31 better than does thecompensation function by the phase difference plates 32 and 33 alone.The viewing-angle dependent coloration of the liquid crystal screen isespecially restrained better. Consequently, such a liquid crystaldisplay device, including the phase difference plates 32 and 33 and theliquid crystal display element 31 can restrain the reversion phenomenon,the decrease in contrast ratio in the opposite viewing direction, andthe coloration phenomenon.

The aforementioned range is, more specifically, such thatΔn(450)−Δn(650), i.e. the difference between the refractive indexanisotropy Δn(450) of the liquid crystal material for rays of lighthaving the wavelength of 450 nm and the refractive index anisotropyΔn(650) thereof for rays of light having the wavelength of 650 nm, isnot less than 0 and less than 0.0090. A more preferred range is suchthat Δn(450)−Δn(650) is not less than 0 and not more than 0.0045.

Especially, by specifying Δn(450)−Δn(650) to be not less than 0 and lessthan 0.0090, the resultant liquid crystal display device, exhibitingcoloration at the viewing angle of 50° which is typically required forliquid crystal display devices, however, achieves well restrainedcoloration to the extent that is up to standard for real use for anyviewing direction.

Moreover, by specifying Δn(450)−Δn(650) to be not less than 0 and notmore than 0.0045, the resultant liquid crystal display device can carryout display that is totally free from the coloration phenomenon for anyviewing direction at the viewing angle of 70° which is typicallyrequired for wide viewing-angle liquid crystal display devices.

As described here, the above-mentioned configuration can remarkablyimprove the quality of the images displayed by the liquid crystaldisplay device, since the contrast ratio in black-and-white display isnot affected by the viewing direction of a viewer.

Besides, in a liquid crystal display device having the aforementionedbasic configuration and such a liquid crystal display device thatΔn(450)−Δn(650) is set to be not less than 0 and less than 0.0090, sincethe refractive index anisotropy Δn(550) of the liquid crystal materialfor rays of light having the wavelength of 550 nm is set to be largerthan 0.060 and smaller than 0.120, the phase difference that occurs tothe liquid crystal display element 31 according to the viewing angle iseliminated. This is, as mentioned in the first embodiment, based on theobservations of decreases in the reversion phenomenon and contrast ratiofor some viewing directions when the refractive index anisotropy Δn(550)of the liquid crystal material for rays of light having the wavelengthof 550 nm is set to be not more than 0.060 or not less than 0.120.Therefore, the contrast variations and reversion phenomenon in the leftand right directions, as well as the coloration phenomenon caused by theviewing-angle dependence, can be further restrained on the liquidcrystal screen.

This eliminates the phase difference that happens to the liquid crystaldisplay element depending upon the viewing angle. Therefore, thecontrast variations and reversion phenomenon in the left and rightdirections, not to mention the coloration phenomenon caused by theviewing-angle dependence, can be further restrained on the displayscreen.

With the liquid crystal display device thus configured, if therefractive index anisotropy Δn(550) of the liquid crystal material forrays of light having the wavelength of 550 nm is set to be not less than0.070 and not more than 0.095, the contrast variations caused by theviewing-angle dependence and reversion phenomenon in the left and rightdirections can be even further restrained.

In the liquid crystal display devices incorporating the aforementionedbasic configuration, (n_(a)−n_(b))×d, i.e. the difference between theprincipal refractive indices n_(a) and n_(b) multiplied by the thicknessd of the phase difference plates, is preferably specified in the rangefrom 80 nm to 250 nm. With such a specification, it becomes possible toensure the compensation function of phase difference by the phasedifference plates 32 and 33. Consequently, the visibility of the liquidcrystal display device can be surely improved.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art intended tobe included within the scope of the following claims.

1. A liquid crystal display device, comprising: a liquid crystal displayelement including: a pair of substrates, a liquid crystal layersandwiched by said substrates; a pair of polarizers disposed so as tosandwich said liquid crystal display element; and at least one phasedifference plate, each of said at least one phase difference platedefining a surface and being disposed between said liquid crystaldisplay element and said pair of polarizers; wherein (i) each of said atleast one phase difference plate has three principal refractive indicesn_(a), n_(b), and n_(c), (ii) said refractive indices are mutuallyrelated by the inequality n_(a)<n_(b)<n_(c), (iii) the direction of saidprincipal refractive index n_(a) coincides with the direction of ay-coordinate axis among x and y-coordinate axes on said surface, saidy-coordinate axis being orthogonal to said normal, and (iv) thedirection of said principal refractive index n_(b) inclines relative tothe normal to said surface and to the direction of said x-coordinateaxis, and wherein the refractive index anisotropy Δn (550) of saidliquid crystal material for rays of light having the wavelength of 550nm is specified to be more than 0.060 and less than 0.120, and whereinthe refractive index anisotropy of said liquid crystal material varieswith other wavelengths of rays of light within a range that allowssubstantially no viewing angle dependent coloration to occur in an imagedisplayed by said device.
 2. The liquid crystal display device asdefined in claim 1, wherein the refractive index anisotropy Δn (550) isspecified to be not less than 0.065 and not more than 0.115.
 3. Theliquid crystal display device as defined in claim 2, wherein therefractive index anisotropy Δn(550) is specified to be not less than0.070 and not more than 0.095.
 4. The liquid crystal display device asdefined in claim 1, wherein the inclination angle of the principalrefractive index n_(b) of the phase difference plate is specified to bein the range from 15° to 75°.
 5. A liquid crystal display device,comprising: a liquid crystal display element including: a pair ofsubstrates, a liquid crystal layer sandwiched by said substrates; a pairof polarizers disposed so as to sandwich said liquid crystal displayelement; and at least one phase difference plate, each of said at leastone phase difference plate defining a surface and being disposed betweensaid liquid crystal display element and said pair of polarizers; whereineach of said at least one phase difference plate (i) has three principalrefractive indices n_(a), n_(b), and n_(c), (ii) said refractive indicesare mutually related by the inequality n_(a)<n_(b)<n_(c), (iii) thedirection of said principal refractive index n_(a) coincides with thedirection of a y-coordinate axis among x and y-coordinate axes on saidsurface, said y-coordinate axis being orthogonal to said normal, and(iv) the direction of the principal refractive index n_(b) inclinesrelative to the normal to said surface and to the direction of saidx-coordinate axis, and wherein (i) the refractive index anisotropy Δn(550) of the liquid crystal material for rays of light having thewavelength of 550 nm is specified to be more than 0.060 and less than0.120, (ii) Δn (450)−Δn (650), i.e., the difference between therefractive index anisotropy Δn (450) of the liquid crystal material forrays of light having a wavelength of 450 nm and the refractive indexanisotropy Δn (650) thereof for rays of light having the wavelength of650 nm, is specified to be not less than 0.0070 and not more than0.0250, and (iii) the refractive index anisotropy of said liquid crystalmaterial varies with other wavelengths of rays of light within a rangethat allows substantially no viewing angle dependent coloration to occurin an image displayed by said device.
 6. The liquid crystal displaydevice defined in claim 5, wherein Δn(450)−Δn(650) is specified to benot less than 0.0200 and not more than 0.0250.
 7. The liquid crystaldisplay device as defined in claim 5, wherein the inclination angle ofthe principal refractive index n_(b) of the phase difference plate isspecified to be in the range from 15° to 75°.
 8. The liquid crystaldisplay device as defined in claim 7, wherein the optical phasedifference plate includes: a support base composed of a transparentorganic high polymer; and a liquid crystal polymer layer formed on thesupport base to be aligned to possess hybrid orientation andcrosslinked.
 9. The liquid crystal display device as defined in claim 5,wherein the refractive index anisotropy Δn (550) is specified to be notless than 0.065 and not more than 0.115.
 10. The liquid crystal displaydevice as defined in claim 9, wherein the refractive index anisotropy Δn(550) is specified to be not less than 0.070 and not more than 0.095.11. The liquid crystal display device as defined in claim 5, wherein theoptical phase difference plate includes: a support base composed of atransparent organic high polymer; and a liquid crystal polymer layerformed on the support base to be aligned to possess oblique orientationand crosslinked.
 12. A liquid crystal display device, comprising: aliquid crystal display element including a liquid crystal layersandwiched by a pair of light-transmitting substrates each having anelectrode layer provided thereon; a pair of polarizers disposed so as tosandwich said liquid crystal display element; and at least one phasedifference plate, each said phase difference plate defining a surfaceand being disposed between said liquid crystal display element and saidpair of polarizers, wherein the improvement comprises (i) each of saidat least one phase difference plate having three principal refractiveindices n_(a), n_(b), and n_(c) being mutually related by the inequalityn_(a)<n_(b)<n_(c), the direction of the principal refractive index n_(a)coinciding with the direction of a y-coordinate axis among x andy-coordinate axes on each said surface of said at least one phasedifference plate, the y-coordinate axis being orthogonal to said normal,and the direction of the principal refractive index n_(b) incliningrelative to the normal to said surface and to the direction of saidx-coordinate axis, and, (ii) the refractive index anisotropy Δn (550) ofthe liquid crystal material for rays of light having the wavelength of550 nm being specified to be more than 0.060 and less than 0.120, (iii)Δn (450)−Δn (650), i.e., the difference between the refractive indexanisotropy Δn (450) of the liquid crystal material for rays of lighthaving a wavelength of 450 nm and the refractive index anisotropy Δn(650) thereof for rays of light having the wavelength of 650 nm, beingspecified to be not less than 0.0070 and not more than 0.0250, and (iv)the refractive index anisotropy of said liquid crystal material beingspecified to vary with other wavelengths of rays of light within a rangethat allows substantially no viewing-angle dependent coloration to occuron a displayed image.
 13. A liquid crystal display device, comprising: aliquid crystal display element including: a pair of substrates, a liquidcrystal layer sandwiched between said substrates; a pair of polarizersdisposed so as to sandwich said liquid crystal display element; and atleast one phase difference plate, each said at least one phasedifference plate defining a surface and being disposed between saidliquid crystal display element and said pair of polarizers, wherein (i)each of said at least one phase difference plate has three principalrefractive indices n_(a), n_(b), and n_(c) being mutually related by theinequality n_(a)<n_(b)<n_(c), (ii) the direction of the principalrefractive index n_(a) coincides with the direction of a y-coordinateaxis among x and y-coordinate axes on each said surface of said at leastone phase difference plate, the y-coordinate axis being orthogonal tosaid normal, and (iv) the direction of the principal refractive indexn_(b) inclines relative to the normal to said surface and to thedirection of said x-coordinate axis; wherein Δn (450)−Δn (650), i.e.,the difference between the refractive index anisotropy Δn (450) of theliquid crystal material for rays of light having a wavelength of 450 nmand the refractive index anisotropy Δn (650) thereof for rays of lighthaving the wavelength of 650 nm, is specified to be not less than 0.0070and not more than 0.0250, and wherein the refractive index anisotropy ofsaid liquid crystal material varies with other wavelengths of rays oflight within a range that allows substantially no viewing angledependent coloration to occur in an image displayed by said device. 14.The liquid crystal display device as defined in claim 13, wherein therefractive index anisotropy Δn (550) of the liquid crystal material forrays of light having the wavelength of 550 nm is specified to be morethan 0.060 and less than 0.120.
 15. The liquid crystal display device asdefined in claim 14, wherein the refractive index anisotropy Δn (550) ofthe liquid crystal material for rays of light having the wavelength of550 nm is specified to be not less than 0.065 and smaller than 0.115.16. The liquid crystal display device as defined in claim 13, whereinthe refractive index anisotropy Δn (550) is specified to be not lessthan 0.065 and not more than 0.115.
 17. The liquid crystal displaydevice as defined in claim 13, wherein the refractive index anisotropyΔn (550) is specified to be not less than 0.070 and not more than 0.095.18. The liquid crystal display device as defined in claim 13, wherein Δn(450)−Δn (650) is specified to be not less than 0.0200 and not more than0.0250.
 19. The liquid crystal display device as defined in claim 15,wherein the refractive index anisotropy Δn (550) is specified to be notless than 0.070 and not more than 0.095.
 20. The liquid crystal displayas defined in claim 13, wherein the inclination angle of the principalrefractive index n_(b) of the phase difference plate is specified to bein the range from 15° to 75°.